Carbon brake wear for aircraft

ABSTRACT

A method and means are provided for extending the life of carbon brakes on aircraft. The method comprises measuring the speed of the aircraft and the intensity of braking and comparing these to predetermined maximum values for each. If the values are both lower than the maximum values, one or more of the brakes are selectively disabled.

This invention relates to a method and means for increasing the life ofcarbon aircraft brakes. More particularly, this invention relates to thecontrolled application of braking pressure to only selected brakesduring low speed ground travel.

BACKGROUND

Modern aircraft which are designed to carry large passenger or cargopayloads are often provided with carbon brakes on each of the wing orbody mounted wheels. The nose wheel is typically not braked. Carbonbrakes are preferred because of their light weight and performancecharacteristics and generally comprise a piston housing and parts, atorque plate and a carbon heat sink stack. This stack contains all thefriction surfaces which, when compressed, cause the wheel to decreaseits speed. The stack comprises a pressure plate, rotor disks, statordisks and backing plate. Carbon composite rotors are connected to thewheel through the rotor drive keys and turn with the wheel. Carboncomposite stators, pressure plates and backing plate are connected tothe torque tube and do not turn. Braking friction is caused when therotors are compressed against the stators.

While carbon brakes are preferred for weight and performance reasonsover steel brakes, the cost of replacing the stack divided by the numberof landing cycles between replacements is much higher than for steelbrakes.

Aircraft brake control systems are designed with separate pedal controlsfor the left and right brakes. When one of the brake pedals isdepressed, all the brakes on that side of the aircraft are commanded toapply simultaneously and equally. By applying all brakes equally, theheat energy absorbed by each individual brake is minimized. For steelbrakes, brake life is largely determined by the total amount of energyabsorbed by each brake and is largely unaffected by the number of brakeapplications that accumulate that energy. Hence, brake control systemsthat apply all brakes simultaneously and equally provide economicoperation of steel brakes and minimize exposure to overheating of anyindividual brake. However, direct application of this method to carbonbrakes does not extend and may significantly shorten their lives.Accordingly, this invention provides a novel method and means to extendthe life of carbon brakes and substantially reduce their operating cost.

BRIEF SUMMARY

In accordance with the invention carbon brake life is significantlyextended by decreasing the number of brake applications during eachlanding cycle. More particularly, brake wear has been found to correlatesignificantly with the number of brake applications and to not besignificantly affected by the energy absorbed during each. By far thelargest number of brake applications occur during ordinary taxiing, soin preferred embodiments of this invention, only some of the brakes areapplied in response to brake applications under ordinary taxiingconditions. An alternating wheel braking pattern is established tominimize brake wear at each braked wheel and yet to promote evendistribution of absorbed energy among all the brakes. This, in turn,prevents overheating of any individual brake. The extended brake-wearsystem is activated only when aircraft ground speed and brakeapplication pressures are typical of taxi operations. Preferably,aircraft speed and hydraulic pressure are sensed so that brakes at allwheels will be operative in critical braking situations such as landing,parking, or emergency stopping.

The invention will be better understood in terms of the Figures anddetailed description which follow.

DETAILED DESCRIPTION

FIG. 1 is a simplified schematic of a subsystem for aircraft brakeswhich alternately disables one of two brakes in order to limit thenumber of brake applications and extend carbon brake life.

FIG. 2 is a schematic view of a sixteen wheel and brake landing gearconfiguration for a wide bodied aircraft showing a brake disable circuitwhich would be activated under low braking pressure and aircraft speedconditions representative of taxi braking to disable half the brakes andthereby extend brake life.

For carbon brakes, the landings to wear-out ratio is strongly dependenton the number of brake applications rather than the energy absorbed by abrake during each application. For commercial passenger aircraft, thebrakes may be applied an average of twenty times per landing cycle. Thebrakes are generally applied during landing absorbing several millionfoot-pounds for heavy wide-bodied aircraft and once to stop the wheelsfrom spinning before they are retracted after take-off. Both of theseare “high speed” brake applications, and are typically at moderatehydraulic pressures less than about 1500 psi hydraulic pressure. Thebalance of the brake applications are “taxi snubs” for steering or lowspeed braking. They create hydraulic brake fluid pressures generallyless than about 1500 psi and absorb about 0.5 MFP average per snub forwide-bodied aircraft. These taxi snubs account for a significant amountof brake energy temperature buildup, and for carbon brakes, most of thewear since carbon brake wear is dependent on the number of brakeapplications. Occasionally, “emergency” brake applications may be madeat higher pressures (up to 3000 psi hydraulic fluid pressure), but suchemergency braking is an insignificant wear factor.

Conventional brake wear control systems provide for applying all brakesequally, gently, and simultaneously during normal taxi braking. Inaccordance with a preferred embodiment of this invention, the life ofcarbon brakes is extended by minimizing the number of brake applicationswhile distributing the heat energy absorbed substantially equally amongall the brakes. This is accomplished by alternately applying only aselected number of brakes rather than all the brakes during each normaltaxi braking operation.

A simplified example of a preferred embodiment of this invention isshown schematically in FIG. 1. A left wheel 2 and right wheel 4 are onthe same side of an airplane and are actuated by the one of the twobrake pedals in the cockpit. Wheel 2 has a carbon brake 6 and rightwheel 4 a carbon brake 8. In this embodiment, the antiskid controlsystem 10 is integral to the brake disable system. Left and right wheelspeed sensors 12 and 14, electronically measure wheel speeds and inputthe signals generated to the antiskid control circuit 10. Signals fromantiskid control circuit 10 are outputted through diodes 16 and 18 toleft and right hydraulic antiskid valves 20 and 22. The signals fromwheel speed sensors 12 and 14 are integrated by antiskid control circuit10 and outputted to brake disable control circuit 24.

Brake metering valve 26 which is responsive to a call for braking fromthe cockpit is located in brake hydraulic line 28. The static linepressure is low, pressure during taxi snubs is higher, and pressureduring parking and emergency braking is relatively higher still. Thispressure is measured at metered brake pressure sensor 30. The signalfrom sensor 30 is inputted to brake disable control circuit 24.

The system works in accordance with the invention as follows. The speedsof wheels 2 and 4 are sensed through sensors 12 and 14 and processed inantiskid control circuit 10 to determine aircraft speed. That aircraftspeed signal is inputted to brake disable circuit 24. The desiredintensity of braking action is sensed by the metered brake pressuresensor 30 and is also inputted to brake disable circuit 24. Inside brakedisable control circuit 24, the metered pressure signal is comparedagainst a first predetermined value, 100 psi for example, to detect whena brake application has been commanded. At the moment at which a brakeapplication is detected, a comparison is made between the aircraft speedsignal and a predetermined value for aircraft speed in brake releaselogic circuit 32. If the speed is higher than the predetermined value,40 mph, for example, then brake disablement is not enabled.Subsequently, comparison is continuously made inside brake release logic32 between the metered pressure signal value and a second predeterminedvalue. If the pressure is greater than the second predetermined value,greater than 1500 psi, for example, then the brake disable controlcircuit 24 does not disable any brakes. That is, if heavy brakingintensity is called for, all the brakes are applied. If and only ifaircraft speed at the time of brake application and metered brakepressure are lower than their predetermined maximum values will brakerelease logic circuit 42 be activated.

As indicated by bipolar knife switch 36, only one of the two antiskidvalves 20 and 22 will be commanded to release its respective brakethrough left diode 38 or right diode 40 when brake release logic 32triggers. Brake select logic circuit 42 remembers which brake was lastdisabled and switches switch 36 when a new brake application has beendetected by brake disable circuit 24.

Brake disable logic 24 responds to both the metered pressure signal andthe aircraft speed signal at the time of brake application. Thereafter,logic circuit 24 responds only to the metered pressure signal fromsensor 30 until the metered brake pressure returns to the no-brakingsystem pressure. This ensures that following a high speed brakeapplication, such as a landing, the brake release command will not beproduced, and half the brakes will not be released, as the aircraftdecelerates through the brake disable speed threshold. The disablesignal would then only be produced at low speed after the brakes werereleased, then reapplied.

If an emergency stop, i.e., high metered pressure is sensed by brakedisable circuit 24, then brake release logic 32 removes the brakerelease command so that both brakes 6 and 8 are applied, thus insuringfull aircraft braking capability when it is needed. Similarly, if ahigher speed stop, such as a landing stop or rejected take off, issensed by brake disable circuit 24 from the aircraft speed signal, thenthe brake release logic 32 removes the brake release command so thatboth brakes 6 and 8 may share the braking energy, preventing overheatingof an individual brake or brakes.

While the desired braking intensity has been described in terms ofmetered braking pressure, other input to the brake disable circuitproviding like information would be equally useful. For example, theacceleration and throw of the brake pedal in the cockpit could bemonitored or the rate of brake temperature increase. Similarly, inputother than aircraft speed such as wheel speed or aircraft ground speedmeasured independently of the wheel speed could be inputted to the brakedisable circuit. Such alternatives will be apparent to those skilled inthe art.

The invention has been described specifically in FIG. 1 in terms of abrake pair on one side of an aircraft. However, systems in accordancewith this invention for aircraft with other numbers and arrangements ofcarbon braked wheels could be readily adapted by persons skilled in theart. For example, FIG. 2 shows the wheel configuration for a wide-bodiedBoeing 747-400™ series aircraft equipped with a carbon brake on eachmain gear wheel. The nose wheel which is not braked is not shown.

Referring to FIG. 2, there are four four-wheel trucks located under theleft wing 44, left body 46, right body 48 and right wing 50 of anaircraft. Using truck 44 as an example, wheels 52 and 54 on one side,and 56 and 58 on the other side of a four-wheel axle frame 60 eachprovide input to a brake disable circuit 62 like that described inFIG. 1. A metered brake pressure signal would also be provided to eachlike brake disable circuit. Thus, when both the aircraft speed at timeof brake application and metered brake pressure are below target values,half of the sixteen brakes would be disabled. For example, brakes onwheels 52 and 54 on the left side of the truck 60 would be alternatelydisabled during successive brake applications as would the brakes onwheels 56 and 58.

Since Carbon brake wear is a function of the number of applications, andsince the vast majority of brake applications occur during taxiing, thelife of carbon brakes is significantly improved by practicing thisinvention. For example, if half the brakes are applied during each taxibrake application, brake wear life could nearly double. The life ofcarbon brakes might be proportionately extended even further bydisabling even more than half the brakes during each braking cycle.System logic insures maximum braking capability during emergencybraking, i.e., high pressure, conditions. Overheating of individualbrakes is prevented because system logic alternates between brakes toshare the braking energy among all the brakes.

Other system refinements such as redundant metered pressure sensorscould be added to improve failure mode performance. Also, means could beprovided to smooth brake pedal control responsiveness in the cockpitbetween partial brake and full brake transitions. That is, the backpressure on the brake pedal could be adjusted so that equal pedaldepression results in equal braking responsiveness irrespective of howmany brakes are being disabled at a given time. Brake temperature couldalso be considered in the brake disabling algorithm to preventdisablement if some brakes are too hot from previous brake applications.

While the invention has been described in terms of specific embodimentsthereof, other forms may be readily adapted by one skilled in the art.Accordingly, the scope of the invention is to be limited only inaccordance with the following claims.

The invention in which an exclusive property or privilege is claimed aredefined as follows:
 1. A method of controlling carbon brakes of multiplebrake aircraft, comprising: receiving a signal corresponding to a speedof an aircraft when braking; receiving a signal corresponding to adesired braking intensity; comparing the speed of the aircraft with apreset value for the aircraft speed only at the moment at which a brakeapplication is detected; comparing the desired braking intensity with apreset value for braking intensity repeatedly during braking; and if thespeed is below the preset value for the aircraft speed and the brakingintensity is below the preset value for the braking intensity, directinga signal to disable at least one set of the carbon brakes during brakingand thereafter directing a signal to selectively disable a further setof the carbon brakes during a succeeding brake application.
 2. Themethod of claim 1, further comprising detecting the speed of theaircraft when braking, and transmitting the signal corresponding to thespeed of the aircraft when braking.
 3. The method of claim 1, furthercomprising detecting the desired braking intensity and transmitting thesignal corresponding to the desired braking intensity.
 4. The method ofclaim 1 wherein comparing the desired braking intensity repeatedlyduring braking includes comparing the desired braking intensitycontinuously throughout braking.
 5. The method of claim 1, furthercomprising selectively disabling at least one set of the carbon brakes.6. The method of claim 1 wherein directing a signal to selectivelydisable at least one set of the carbon brakes includes directing asignal to selectively disable half the carbon brakes of the aircraft. 7.The method of claim 1 wherein directing a signal to selectively disableat least one set of the carbon brakes includes directing a signal toselectively disable more than half the carbon brakes of the aircraft. 8.The method of claim 1, further comprising: receiving a signalcorresponding to a brake temperature; and if the brake temperature isabove a predetermined value, preventing disabling of the at least oneset of the carbon brakes.
 9. A system for controlled application ofbraking pressure to only selected brakes during low speed ground travel,comprising: a brake release logic circuit portion; first receiving meansfor receiving a signal corresponding to an aircraft speed; secondreceiving means for receiving a signal corresponding to a metered brakepressure; a brake disable control circuit portion; means for couplingthe signal corresponding to the aircraft speed and the signalcorresponding to the metered brake pressure to an input of the brakedisable control circuit portion that includes comparing means, thecomparing means being configured to: compare the signal corresponding tothe metered brake pressure with a first predetermined value of brakepressure to detect when a brake application has been commanded; when abrake application has been detected, compare the signal corresponding tothe aircraft speed with a predetermined value of aircraft speed; whenthe signal corresponding to the aircraft speed is greater than thepredetermined speed, direct a signal to prevent disablement of anybrakes; repeatedly compare the signal corresponding to the metered brakepressure with a second predetermined value of brake pressure; and whenthe signal corresponding to the metered brake pressure is greater thanthe second predetermined value of brake pressure, direct a signal toprevent disablement of any brakes.
 10. The system of claim 9, furthercomprising means for speed detection positioned to detect the speed ofthe aircraft when braking and transmit the signal corresponding to thespeed of the aircraft when braking.
 11. The system of claim 9, furthercomprising means for braking intensity detection positioned to detectthe metered brake pressure and transmit the signal corresponding to themetered brake pressure.
 12. The system of claim 9 wherein the means forcoupling is configured to compare the signal corresponding to themetered braking pressure continuously throughout braking.
 13. The systemof claim 9 wherein the brake disable control circuit portion isconfigured to selectively disable half the carbon brakes of theaircraft.
 14. The system of claim 9 wherein the brake disable controlcircuit portion is configured to selectively disable more than half thecarbon brakes of the aircraft.
 15. The system of claim 9, furthercomprising a brake temperature sensor positioned to detect a temperatureof the carbon brakes and transmit a signal corresponding to thetemperature, and wherein the means for coupling is configured to receivethe signal corresponding to the temperature and, if the braketemperature is above a predetermined value, prevent disabling of the atleast one set of the carbon brakes.
 16. An apparatus for controllingcarbon brakes for multiple brake aircraft, comprising: a first receivingcircuit portion configured to receive a signal corresponding to anaircraft speed; a second receiving circuit portion configured to receivea signal corresponding to a metered brake pressure; a brake releasecircuit portion configured to direct brake release; a brake disablecircuit portion configured to direct brake disablement; and a comparisoncircuit portion operatively coupled among the first and second receivingcircuit portions, the brake release circuit portion and the brakedisable circuit portion, the comparison circuit portion being configuredto: compare the signal corresponding to the metered brake pressure witha first predetermined value of brake pressure to detect when a brakeapplication has been commanded; when a brake application has beendetected, compare the signal corresponding to the aircraft speed with apredetermined value of aircraft speed; direct a signal to preventdisabling of any brakes when the signal corresponding to the aircraftspeed is greater than the predetermined speed; repeatedly compare thesignal corresponding to the metered brake pressure with a secondpredetermined value of brake pressure; and direct a signal to preventdisabling of any brakes when the signal corresponding to the meteredbrake pressure is greater than the second predetermined value.
 17. Theapparatus of claim 16 further comprising a speed detector positioned todetect the speed of the aircraft when braking and transmit the signalcorresponding to the speed of the aircraft when braking.
 18. Theapparatus of claim 16 further comprising a braking intensity detectorpositioned to detect the metered brake pressure and transmit the signalcorresponding to the metered brake pressure.
 19. The apparatus of claim16 wherein the comparison circuit portion is configured to compare thesignal corresponding to the metered brake pressure continuouslythroughout braking.
 20. The apparatus of claim 16 wherein the brakedisable circuit portion is configured to selectively disable half thecarbon brakes of the aircraft.
 21. A computer-based system forcontrolling carbon brakes of multiple brake aircraft, comprising: afirst receiver portion configured to receive a signal corresponding to aspeed of an aircraft when the aircraft is braking; a second receiverportion configured to receive a signal corresponding to a desired bakingintensity; a first comparer portion configured to compare the signalcorresponding to the speed of the aircraft with a predetermined value atonly at least approximately the moment at which a brake application isdetected; a second compare portion configured to compare the signalcorresponding to the desired braking intensity with a predeterminedvalue for braking intensity repeatedly during braking; and a disablingportion configured to direct disablement of at least one set of thecarbon brakes if the speed is below the predetermined value for theaircraft speed and the braking intensity is below the predeterminedvalue for the braking intensity, the braking portion further beingconfigured to direct disablement of another set of the carbon brakesduring a succeeding brake application.
 22. The system of claim 21,further comprising a speed detector positioned to detect the speed ofthe aircraft when braking and transmit the signal corresponding to thespeed of the aircraft when braking.
 23. The system of claim 21, furthercomprising a braking intensity detector positioned to detect the desiredbraking intensity and transmit the signal corresponding to the desiredbraking intensity.
 24. The system of claim 21 wherein the second compareportion is configured to compare the signal corresponding to the desiredbraking intensity continuously throughout braking.
 25. The system ofclaim 21 wherein the disabling portion is configured to directdisablement of half the carbon brakes of the aircraft.
 26. An aircraft,comprising: a fuselage; a wing depending from the fuselage a propulsionsystem depending from at least one of the wing and the fuselage; alanding gear system having a plurality of carbon brake sets; and acomputer-based system for controlling the carbon brake sets, thecomputer-based system including: a first receiver portion configured toreceive a signal corresponding to a speed of an aircraft when theaircraft is braking; a second receiver portion configured to receive asignal corresponding to a desired baking intensity; a first compareportion configured to compare the signal corresponding to the speed ofthe aircraft with a predetermined value at only at least approximatelythe moment at which a brake application is detected; a second compareportion configured to compare the desired braking intensity with apredetermined value for braking intensity repeatedly during braking; anda disabling portion configured to direct disablement of at least one setof the carbon brakes if the speed is below the predetermined value forthe aircraft speed and the braking intensity is below the predeterminedvalue for the braking intensity, the braking portion further beingconfigured to direct disablement of another set of the carbon brakesduring a succeeding brake application.
 27. The aircraft of claim 26,further comprising a speed detector positioned to detect the speed ofthe aircraft when braking and transmit the signal corresponding to thespeed of the aircraft when braking.
 28. The aircraft of claim 26,further comprising a braking intensity detector positioned to detect thedesired braking intensity and transmit the signal corresponding to thedesired braking intensity.
 29. The aircraft of claim 26 wherein thesecond compare portion is configured to compare the desired brakingintensity continuously throughout braking.
 30. The aircraft of claim 26wherein the disabling portion is configured to direct disablement ofhalf the carbon brakes of the aircraft.